feat: 全新设计系统重构 — 精确极简靛蓝色调

- 全新设计令牌系统: 靛蓝/紫罗兰品牌色(#5e6ad2/#7170ff)
- Ring-based 阴影系统替代传统 box-shadow
- 精确字体排版: font-feature-settings, letter-spacing, weight 510/590
- 完整暗色主题支持: 所有颜色令牌均有暗色覆盖
- 新增组件: badge, pill, pill-chip, search-bar, section-label
- 修复 .sidebar 类名冲突(导航栏 vs 产品详情侧栏)
- 消除所有内联样式, 统一使用 CSS 类
- 修复无效 prefetch 链接(style.css → main.css)
- 移除所有硬编码颜色值, 统一使用 CSS 自定义属性

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Digidai

_layouts/default.html

@@ -5,8 +5,8 @@
     <meta name="viewport" content="width=device-width, initial-scale=1.0">
     <meta name="referrer" content="strict-origin-when-cross-origin">
     <meta name="color-scheme" content="light dark">
-    <meta name="theme-color" content="#ffffff" media="(prefers-color-scheme: light)">
-    <meta name="theme-color" content="#191919" media="(prefers-color-scheme: dark)">
+    <meta name="theme-color" content="#f7f8f8" media="(prefers-color-scheme: light)">
+    <meta name="theme-color" content="#0f1011" media="(prefers-color-scheme: dark)">
 
     <!-- Search Engine Verification -->
     {% include search-engine-verification.html %}
@@ -81,7 +81,7 @@
 
     <!-- Performance and Resource Hints -->
     <link rel="prefetch" href="{{ site.baseurl }}/product-directory/">
-    <link rel="prefetch" href="{{ site.baseurl }}/assets/css/style.css">
+    <link rel="prefetch" href="{{ site.baseurl }}/assets/css/main.css">
 
     {%- comment -%} 预加载关键资源 {%- endcomment -%}
     <link rel="preload" href="{{ site.baseurl }}/assets/images/logo.png" as="image">

_sass/_base.scss

@@ -1,4 +1,7 @@
+// ═══════════════════════════════════════════════
 // Base Styles & Reset
+// ═══════════════════════════════════════════════
+
 * {
     margin: 0;
     padding: 0;
@@ -10,15 +13,17 @@ html {
 }
 
 body {
-    font-family: 'Inter', -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif;
+    font-family: 'Inter', -apple-system, system-ui, 'Segoe UI', Roboto, Oxygen, Ubuntu, Cantarell, sans-serif;
     font-size: 15px;
+    font-weight: 400;
     line-height: 1.6;
     color: var(--text-primary);
-    background: var(--bg-primary);
+    background: var(--bg-page);
     display: flex;
     min-height: 100vh;
     -webkit-font-smoothing: antialiased;
     -moz-osx-font-smoothing: grayscale;
+    font-feature-settings: "cv01" 1, "ss03" 1;
 }
 
 // Prevent iOS zoom on input focus